The present invention relates to electrodeless high-intensity discharge (HID) lamps and, more particularly, to novel means with piezoelectrically-located electrodes for initiating a plasma discharge within the arc space of the electrodeless HID lamp.
It is now well known to provide a toroidal light-emitting plasma within the envelopes of a HID lamp. The induction arc plasma depends upon a solenoidal, divergence-free electric field for its maintenance; the field is created by the changing magnetic field of an excitation coil, which is typically in the form of a solenoid. It is necessary to develop a very high electric field gradient across the arc tube to start the plasma discharge; it is difficult to develop a sufficiently high electric field gradient, especially in the associated excitation coil, because the coil current may be prohibitively high, even if it is to be provided only on a pulse basis. Further, providing a very high electric field gradient may be impossible because the necessary field-per-turn of the excitation coil may exceed the turn-to-turn electrical breakdown reading of that coil. Thus, it is difficult to provide some means for starting induction-driven HID lamps, and it is also difficult to provide for hot restarting of the same type of lamp. It is therefore highly desirable to provide some means for starting the HID lamp plasma discharge, which starting means can be easily utilized with typical HID lamps, under normal ambient conditions. One type of starting electrode, and a heat-sensitive means for moving those electrodes into and out of a starting location, are described and claimed in U.S. application Ser. No. (225,315), filed July 28, 1988, assigned to the assignee of the present invention and incorporated herein by reference in its entirety. However, it is desirable to be able to selectively move the starting electrodes, so as to be able to re-start the lamp even if the extinguished arc tube is still hot.